PROJECT SUMMARY Signals from peripheral tissues play an important role in aligning sleep-wake activity with the metabolic, nutri- tional and immune status of the organism. The intestinal microbiota is an essential source of such signals through the production of bacterial metabolites, e.g., short-chain fatty acids (SCFAs), and the release of bacterial cell wall components, lipopolysaccharide (LPS) and peptidoglycans. The long-term objective is to identify mechanisms of microbiota-brain communications and its relevance to sleep regulation. The objective of the present proposal is to investigate the newly discovered sleep-promoting viscerosensory mechanism in the hepatoportal region. Activation of hepatoportal sensors by SCFAs and bacterial cell wall products is a potent sleep-promoting signal. The central hypothesis is that bacterial products, such as LPS that translocate from the intestinal lumen to the liver via the portal vein, activate hepatic macrophages. Macrophages, in turn, secrete prostaglandins locally, which activate the sensory neurons of the hepatic vagus. Vagus carries somnogenic signal to the nucleus tractus solitarius, a component of brain stem sleep circuits. In three specific aims, we will 1) determine the role of hepatic macrophages in LPS-induced sleep in macrophage-depleted rats, 2) determine the contribution of hepatic prostaglandin E2 in sleep signaling 3) determine the contribution of hepatic vagal afferents in hepatoportal sleep induction. The concept of microbiota-gut-brain axis is viewed as a major paradigm shift in neuroscience. Changes in the composition of microbiota as well as increased translocation of microbial products to the systemic circulation are related to pathological conditions, including disorders of the central nervous system. Identifying the role of bacterial products in sleep regulation is important because the gut flora is susceptible to changes in diet, environment, food additives and antibiotic treatment, which could lead to altered sleep, but it can also provide an easily accessible target for translational research to improve sleep.